1. Field of the Invention
The invention concerns the making of an electrical contact to a polycrystalline layer of a semiconductor device.
2. Description of the Prior Art
A significant problem in the fabrication of semiconductor devices involves forming an electrical contact to an external circuit. By necessity to make such an electrical contact, a conducting material, one exhibiting metallic conduction properties, must be applied to a semiconductor material. Often this conductor-semiconductor interface produces a rectifying diode, i.e., a Schottky barrier. This happens, for example when InP is contacted with graphite or molybdenum. (see K. J. Bachmann, E. Buehler, J. L. Shay and S. Wagner, Applied Physics Letters 29, 121 (1976).) For most device applications formation of a barrier at a contact results in two significant problems. First, an unacceptably large voltage is typically needed to break down the barrier and second, even once the barrier is overcome it also often functions in effect as a unacceptably large resistance in series with the device. Therefore, for a more efficient device this blocking voltage and high resistance must usually be significantly reduced. For example blocking voltages less than 0.05 volts and specific resistances below 2 ohm-cm.sup.2 are desirable for common applications of polycrystalline devices.
Various techniques have been developed to minimize the discussed contact problems. The most common methods entail alloying or diffusing a dopant into the surface region of a semiconductor material to produce a transition region between the area of the semiconductor region to be contacted and the active portion of the same region. The highly doped area approaches the conductive properties of the contact and acts as an effective intermediary. There are, however, a number of potential difficulties connected with heavily doping a semiconductor region. The doping must be sufficient to provide the conductivity properties necessary for an effective intermediary area. This level is often very high and can produce chemical deterioration of the semiconductor material or a change in the material's crystal structure, either effect resulting in degraded device characteristics. Additionally at high levels the dopant may segregate additional phases within the semiconductor, also producing a degradation.
The possible problems of doping by diffusion or alloying are compounded when the semiconductor material involved is a polycrystalline thin film. Because of the numerous grain boundaries in a polycrystalline thin layer, the depth of diffusion or alloying of a dopant into the semiconductor is more difficult to control. The relatively small thickness of such a film, typically between 0.5 and 20 micrometers, only enhances the difficulty. If too high a dopant level is introduced in the active area of the semiconductor--the area near the active junction--undesirable excessive electrical currents are produced during device operation. Indeed, in an extreme but not entirely unusual case, a short circuit is formed across the intended active junction. This latter result most often occurs when the dopant used is a fast diffuser in the particular semiconductor being doped. Exemplary of this possibility is the diffusion properties of zinc in indium phosphide. (See A. Hooper, B. Tuck and A. J. Baker, Solid State Electronics, 17, 531 (1974); L. L. Chang and H. C. Casey Solid State Electronics, 7, 481 (1964).)
The utilization of thin polycrystalline film semiconductor regions in the device also adds other complications. Clearly a thin semiconductor film has no substantial structural integrity and must be supported. Additionally, in a thin film device each layer must be grown on the adjacent layer with sufficient adhesion to maintain the necessary structural and electrical properties. For example, presently, although it is easy to produce good InP/CdS junctions by growing CdS on InP (see copending application Ser. No. 718,386 filed Aug. 27, 1976) the reverse growing sequence is more difficult. Thus, it is preferable to first deposit a layer of InP on a conducting substrate and then in sequence to deposit a layer of CdS on the InP. This requires a conducting substrate which offers adequate structural support for the device and upon which InP adheres without flaking or separation. Added to these structural and adhesion requirements are the electrical contact requisites previously discussed for an improved device.
For certain thin film devices, up to now, all the adhesion, structural, and contact requirements could not be entirely satisfied. Exemplary of this situation are InP/CdS thin film devices. As discussed, it is desirable to first put the InP layer on a conducting substrate. An InP layer deposited by a hydrogen transport chemical vapor deposition (CVD) process adheres to molybdenum or graphite substrates. (See Bachmann et al, Appl. Phys. Letts., 29, 121 (1976).) However the substrate-indium phosphide interface has not entirely satisfactory electrical characteristics. (See the Bachmann et al, supra.) The contact problem to the substrate, e.g., graphite or molybdenum, is not amenable to the usual diffusion or alloying solution. Since CdS is preferably put on InP, the thin film InP layer is first deposited on a conducting substrate. However this procedure makes the side of the InP which requires alloying inaccessible. Therefore conventional techniques are inadequate.
The inadequacies of conventional techniques are particularly significant in solar cell devices. In such applications if a high resistance or blocking voltage contact cannot be reduced to suitable levels, the efficiency obtainable from the device is limited. It is apparent that a method of making a low resistance contact to polycrystalline semiconductor films, which does not involve alloying or diffusion and which satisfies the necessary adhesion, structural and electrical properties is quite desirable for some applications e.g., solar energy conversion.